[Haskell] return?

Thanks you guys. I really love this mail list. Can always learn nice tips
from experienced people here. :-)
I guess I'll go with the guard suggestion. ContT is nice, but don't want to
introduce too much extras to the program just for syntax reason.
I do agree with you, Graham. Actually
do
do
earlyreturn 1
return 2
return 3
is equivalent to:
do
earlyreturn 1
return 2
return 3
While the first should be 3 by intuition. the second should be 1.
I guess that's the reason why imperative return is not possible in monad.
Graham Klyne
<GK at ninebynine.or To: Ben_Yu at asc.aon.com, haskell at haskell.org
g> cc:
Subject: Re: [Haskell] return?
04/30/2004 04:45
AM
>Is this possible at all?
I don't think so, in the form that you suggest.
Ultimately, it all comes down to function applications, for which there is
no such "bail out". Rather, I think something like this is required:
do
{ ...
; if cond then return 1
else do
(the rest)
}
Here's an example from some real (tested) code:
[[
-- Open and read file, returning its handle and content, or Nothing
-- WARNING: the handle must not be closed until input is fully evaluated
repOpenFile :: String -> RepStateIO (Maybe (Handle,String))
repOpenFile fnam =
do { (hnd,hop) <- lift $
if null fnam then
return (stdin,True)
else
do { o <- try (openFile fnam ReadMode)
; case o of
Left e -> return (stdin,False)
Right h -> return (h,True)
}
; hrd <- lift $ hIsReadable hnd
; res <- if hop && hrd then
do {
; fc <- lift $ hGetContents hnd
; return $ Just (hnd,fc)
}
else
do { lift $ hClose hnd
; repError ("Cannot read file: "++fnam) 3
; return Nothing
}
; return res
}
]]
#g
--
>Hi,
>While writing monad programs, I sometimes want to do a return as it is in
>imperative program. i.e.,
>do{return 1; return 2} is same as return 1
>>This seems useful to me when I need to do something like
>do
> mwhen cond $ return 1
> ...... -- subsequent actions
>>>I know I can do
>if cond then return 1 else (
> ...--subsequent actions
> )
>>> However, that syntax does not look very pleasant to me due to this extra
>indentation and the pair of parens.
>>>Is this possible at all?
>>Ben.
>>_______________________________________________
>Haskell mailing list
>Haskell at haskell.org>http://www.haskell.org/mailman/listinfo/haskell
------------
Graham Klyne
For email:
http://www.ninebynine.org/#Contact